Fix API requests for branch names ending in .txt

Similar to gitlab-foss!13174 for files, Grape would try to return a
text result when the branch name ended in '.txt'. Fix this by forcing it
to always return JSON results for branches, no matter what the name.

Fixes: #217097

changelogs/unreleased/fix-branch-dot-txt.yml

+---
+title: Fix API requests for branch names ending in .txt
+merge_request: 31446
+author: Daniel Stone
+type: fixed

lib/api/branches.rb

@@ -8,6 +8,8 @@ module API
 
     BRANCH_ENDPOINT_REQUIREMENTS = API::NAMESPACE_OR_PROJECT_REQUIREMENTS.merge(branch: API::NO_SLASH_URL_PART_REGEX)
 
+    after_validation { content_type "application/json" }
+
     before do
       require_repository_enabled!
       authorize! :download_code, user_project

spec/requests/api/branches_spec.rb

@@ -16,6 +16,7 @@ describe API::Branches do
 
   before do
     project.add_maintainer(user)
+    project.repository.add_branch(user, 'ends-with.txt', branch_sha)
   end
 
   describe "GET /projects/:id/repository/branches" do
@@ -240,6 +241,12 @@ describe API::Branches do
         it_behaves_like 'repository branch'
       end
 
+      context 'when branch contains dot txt' do
+        let(:branch_name) { project.repository.find_branch('ends-with.txt').name }
+
+        it_behaves_like 'repository branch'
+      end
+
       context 'when branch contains a slash' do
         let(:branch_name) { branch_with_slash.name }